Security key generation and management method of PDCP distributed structure for supporting dual connectivity

ABSTRACT

The present disclosure relates to a pre-5 th -Generation (5G) or 5G communication system to be provided for supporting higher data rates Beyond 4 th -Generation (4G) communication system such as Long Term Evolution (LTE). A method for communicating by a user equipment with a macro cell base station and a small cell base station in a communication system is provided. The method comprises applying a first base station security key to a first communication link with the macro cell base station; generating a second base station security key to be used for a second communication link with the small cell base station based on the first base station security key; applying the second base station security key to the second communication link with the small cell base station; and communicating through at least one of the first communication link and the second communication link.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation application of U.S. patentapplication Ser. No. 15/895,633, filed Feb. 13, 2018, which will issueas U.S. Pat. No. 10,142,299 on Nov. 27, 2018, which is a continuationapplication of U.S. patent application Ser. No. 14/910,580, filed onFeb. 5, 2016, which has issued as U.S. Pat. No. 9,930,016 on Mar. 27,2018, which was the National Stage of an International applicationnumber PCT/KR2014/007312, filed on Aug. 7, 2014, and was based on andclaimed the benefit of a Korean patent application number10-2013-0094952, filed on Aug. 9, 2013 in the Korean IntellectualProperty Office, the disclosure of each of which is hereby incorporatedby reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to wireless communications. Moreparticularly, the present disclosure relates to methods and apparatusesfor generating and maintaining security keys for a plurality of basestations connected to one terminal in a wireless communication systemsupporting a plurality of wireless link connections to one terminal.

BACKGROUND

To meet the demand for wireless data traffic having increased sincedeployment of 4G (4^(th)-Generation) communication systems, efforts havebeen made to develop an improved 5G (5^(th)-Generation) or pre-5Gcommunication system. Therefore, the 5G or pre-5G communication systemis also called a ‘beyond 4G network’ or a ‘post LTE system’.

The 5G communication system is considered to be implemented in higherfrequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higherdata rates. To decrease propagation loss of the radio waves and increasethe transmission distance, the beamforming, massive multiple-inputmultiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna,an analog beam forming, large scale antenna techniques are discussed in5G communication systems.

In addition, in 5G communication systems, development for system networkimprovement is under way based on advanced small cells, cloud radioaccess networks (RANs), ultra-dense networks, device-to-device (D2D)communication, wireless backhaul, moving network, cooperativecommunication, coordinated multi-points (CoMP), reception-endinterference cancellation and the like.

In the 5G system, hybrid FSK and QAM modulation (FQAM) and slidingwindow superposition coding (SWSC) as an advanced coding modulation(ACM), and filter bank multi carrier (FBMC), non-orthogonal multipleaccess (NOMA), and sparse code multiple access (SCMA) as an advancedaccess technology have been developed.

Installing multiple small cells is being researched to increase thewireless network capability of a macro cell to respond to soring mobiledata traffic.

Small cells with small cell coverage may recycle limited frequencyresources and enables a high data rate of data transmission and transmitpower savings since small cell base stations are positioned relativelyclose to users. The nature of the small cell base station having smallcell coverage may cause frequent handover and radio link failure. Ascheme attracting attention to address such issue is the dualconnectivity that may allow a terminal to maintain a connection with thebase station of the macro cell while simultaneously receiving datathrough a small cell link at a high data rate.

Presumably, small cell base stations may suffer from weak security ascompared with macro cell base stations. Such assumption comes from thefact that, while macro cell base stations are directly managed by thecommunication network provider, small cell base stations scatteredindoor (or in a home) are confronted with difficulty in physicalmanagement for security maintenance.

The dual connectivity may allow a macro cell base station to play a roleas an anchor for controlling multiple small cell base stations. Thus, ifthe security information regarding the macro cell base station isexposed through the small cell base stations, personal informationleaks, illegal billing, or other security issues are more likely tohappen.

Further, assuming a few tens or a few hundreds of small cells to beinstalled to increase network cell capacity, control overhead andlatency issues may arise due to procedures such as security key requestand response ensuing when receiving the respective security keys of thesmall cells from a higher network (e.g., a mobility management entity(MME)).

Therefore, there is a need for a scheme and procedure for effectivelygenerating and managing an independent security key by a layered networkhaving a macro cell and multiple small cells.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present disclosure.

SUMMARY

Aspects of the present disclosure are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentdisclosure is to provide a method and apparatus for generating andmaintaining a security key for a plurality of base stations connected toa single terminal in a wireless communication system. In particular, theinstant disclosure provides a method and apparatus for generating andmaintaining a security key in a wireless communication system under adual connectivity environment, i.e., under the circumstance where aterminal is simultaneously linked to a macro cell base station and asmall cell base station.

Another aspect of the present disclosure describes a scheme for applyingand operating separate security keys for the respective packet dataconvergence protocol (PDCP) layers of a macro cell and a small cell, ifthe macro cell and the small cell have the PDCP layers, in a networkwhere a terminal is simultaneously linked to the macro cell base stationand the small cell base station that coexists in the coverage of themacro cell.

Another aspect of the present disclosure is to provide a method andapparatus for generating and managing an independent security keybetween a macro cell and a small cell layered network in a wirelesscommunication system simultaneously supporting multiple transmissionlinks.

Another aspect of the present disclosure provides an apparatus andmethod for generating, removing, or exchanging security keys when a basestation is connected, released, or exchanged considering the networklayer of a source base station and a target base station due to an issuethat arises owing to a difference in security capacity between per-layerbase stations in the layered network.

Another aspect of the present disclosure provides a process forgenerating a security key and transmitting security key-relatedinformation when a small cell is added to a macro cell base station anddata radio bearer (DRB) starts to be transmitted, when a small cell ischanged (e.g., another small cell is connected), or when a small cell isreleased so that the macro cell resumes serving a corresponding DRB.

Another aspect of the present disclosure provides an apparatus andmethod for independently maintaining the security keys of the macro celland the small cells while selectively maintaining the independence ofsecurity keys between the small cells in order to address the controloverhead and latency issues when independently generating security keysfrom a higher network (a mobility management entity (MME)).

Another aspect of the present disclosure provides an apparatus andmethod for addressing the control overhead and latency issues in such away as to simultaneously generate multiple security keys whenindependently generating security keys from a higher network (a MME).

Another aspect of the present disclosure provides an apparatus andmethod for continuously maintaining a count value that is information toidentify user data forwarded between base stations upon handover througha radio resource control (RRC) reconfiguration process and using thesame PDCP configuration to prevent data loss when changing a connectedbase station (adding, releasing, or exchanging).

In accordance with another aspect of the present disclosure, a methodfor communicating by a user equipment with a macro cell base station anda small cell base station in a communication system is provided. Themethod includes applying a first base station security key to a firstcommunication link with the macro cell base station; generating a secondbase station security key to be used for a second communication linkwith the small cell base station based on the first base stationsecurity key; applying the second base station security key to thesecond communication link with the small cell base station; andcommunicating through at least one of the first communication link andthe second communication link.

In accordance with another aspect of the present disclosure, a methodfor communicating by a macro cell base station with a user equipment anda small cell base station in a communication system is provided. Themethod includes determining a first base station security key for afirst communication link with the user equipment; generating a secondbase station security key to be used for a second communication linkbetween the small cell base station and the user equipment based on thefirst base station security key; and transmitting the generated secondbase station security key to the small cell base station.

In accordance with another aspect of the present disclosure, a userequipment communicating with a macro cell base station and a small cellbase station in a communication system is provided. The user equipmentincludes a controller configured to apply a first base station securitykey to a first communication link with the macro cell base station,generate a second base station security key to be used for a secondcommunication link with the small cell base station based on the firstbase station security key, and apply the second base station securitykey to the second communication link with the small cell base station;and a transceiver configured to communicate through at least one of thefirst communication link and the second communication link.

In accordance with another aspect of the present disclosure, a macrocell base station communicating with a user equipment and a small cellbase station in a communication system is provided. The macro cell basestation includes a controller configured to determine a first basestation security key for a first communication link with the userequipment, and generate a second base station security key to be usedfor a second communication link between the small cell base station andthe user equipment based on the first base station security key; and atransceiver configured to transmit the generated second base stationsecurity key to the small cell base station.

In accordance with another aspect of the present disclosure, a method ofperforming communication by a user terminal forming a communication linkfor data transmission with a macro cell base station and a small cellbase station located in a communication system is provided. The methodincludes applying a first base station security key to a communicationlink with the macro cell base station, generating a second base stationsecurity key to be used for a communication link with the small cellbase station, applying the second base station security key to thecommunication link with the small cell base station, and communicatinguser data through the communication links to which the security keys areapplied.

In accordance with another aspect of the present disclosure, a methodfor performing communication by a macro cell base station forming acommunication link with a user terminal in a communication system isprovided. The method includes a small cell base station and the userterminal, comprising: determining to add a small cell base station toform a communication link with the user terminal, sending a request fora next hop (NH) and a next hop chaining counter (NCC) to a MME,receiving a response including the NCC, generating a first base stationsecurity key to be used for a communication link between the added smallcell base station and the user terminal using the NH included in thereceived response, and transmitting the generated first base stationsecurity key to the added base station.

In accordance with yet another aspect of the present disclosure, a userterminal forming a communication link for data transmission with a macrocell base station and a small cell base station located in acommunication system is provided. The user terminal includes acontroller configured to apply a first base station security key to acommunication link with the macro cell base station, to generate asecond base station security key to be used for a communication linkwith the small cell base station, and to apply the second base stationsecurity key to the communication link with the small cell base station,and a transceiver configured to communicate user data through thecommunication links to which the security keys are applied.

In accordance with still another aspect of the present disclosure,proposes a macro cell base station forming a communication link with auser terminal in a communication system including a small cell basestation and the user terminal is provided. The macro cell base stationincludes a controller configured to determine to add a small cell basestation to form a communication link with the user terminal, to send arequest for a NH and a NCC to a MME, to receive a response including theNCC, and to generate a first base station security key to be used for acommunication link between the added small cell base station and theuser terminal using the received NH, and a transceiver configured totransmit the generated first base station security key to the added basestation.

Another aspect of the present disclosure, the PDCP layer exists in eachof the macro cell and the small cells and separate security keys applyand operate for them in a network where the small cells coexist withinthe coverage of the macro cell so that two or more links aresimultaneously connected to the terminal. Thus, the security of themacro cell base station may be maintained even when using the small cellbase stations with relatively weak security as compared with the macrocell base station.

Further, according to another aspect of the present disclosure, there isprovided a method for generating security keys or transmittinginformation relating to the same under the circumstance where a smallcell is added to the macro cell base station so that a DRB starts to betransmitted or the small cell is released so that the macro cell resumesserving the corresponding DRB. Thus, security keys for multiple smallcells may be generated and control-related overhead issues may beaddressed while maintaining the security of the macro cell base stationfor the small cell base stations. That is, an aspect of the presentdisclosure is to minimize security key control-related overhead whilemaintaining the security for an interface (X2) between the macro cellbase station and a small cell base station with a relatively weaksecurity as compared with the macro cell base station.

Other aspects, advantages, and salient features of the disclosure willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses various embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of certainembodiments of the present disclosure will be more apparent from thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a view illustrating a case in which a terminal establishesdual connectivity for a macro cell and small cells that coexist in awireless communication system according to an embodiment of the presentdisclosure;

FIG. 2 is a view illustrating a network control plane and user planeunder a dual connectivity situation where a macro cell and a small cellare simultaneously connected to a terminal in a wireless communicationsystem according to an embodiment of the present disclosure;

FIG. 3 is a view illustrating an example of a protocol stack structureto support dual connectivity according to an embodiment of the presentdisclosure;

FIG. 4 is a view illustrating another example of a protocol stackstructure to support dual connectivity according to an embodiment of thepresent disclosure;

FIG. 5 is a view illustrating another example of a protocol stackstructure to support dual connectivity according to an embodiment of thepresent disclosure;

FIG. 6 is a view illustrating an example of communication between amacro cell base station, a small cell base station, and a user terminalin a protocol stack structure where a macro cell and a small cell havean independent packet data convergence protocol. (PDCP) layer accordingto an embodiment of the present disclosure;

FIG. 7 is a view illustrating a structure of a security key used in a3rd Generation Partnership Project (3GPP) long term evolution (LTE)system according to an embodiment of the present disclosure;

FIG. 8A is a view illustrating an example of generating a security keyand transmitting related information when handover occurs between basestations in a communication system according to an embodiment of thepresent disclosure;

FIG. 8B is a flowchart illustrating an example of generating a securitykey and transmitting related information when handover occurs betweenbase stations in a communication system according to an embodiment ofthe present disclosure;

FIG. 9A is a view illustrating an example of generating a security keyand transmitting related information for maintaining a separate securitykey when a small cell is added according to an embodiment of the presentdisclosure;

FIG. 9B is a flowchart illustrating an example of generating a securitykey and transmitting related information for maintaining a separatesecurity key when a small cell is added according to an embodiment ofthe present disclosure;

FIG. 10 is a view illustrating a process of generating and transmittinga security key and an initial setup process of a small cell link when amacro cell determines to add a small cell on the control plane accordingto an embodiment of the present disclosure;

FIG. 11 is a view illustrating an process of discarding a security keyand an process in which a terminal resumes communication with a macrocell base station when the macro cell determines to release a small cellon the control plane according to an embodiment of the presentdisclosure;

FIG. 12 is a view illustrating an procedure in which a base stationobtains information for generating a security key from an MME accordingto an embodiment of the present disclosure;

FIG. 13 is a view illustrating another procedure in which a base stationobtains information for generating a security key from an MME accordingto an embodiment of the present disclosure;

FIG. 14 is a view illustrating an process of generating security keysfor a macro cell base station, a small cell base station, and a userterminal and applying the generated security keys when the macro cellbase station determines to add a small cell according to an embodimentof the present disclosure;

FIG. 15 is a view illustrating an process of generating security keysfor a macro cell base station, a small cell base station, and a userterminal and applying the generated security keys when the macro cellbase station determines to release or replace a small cell according toan embodiment of the present disclosure;

FIGS. 16A and 16B are views illustrating an example of generating andtransferring a security key when adding a small cell base station, whenchanging a small cell base station, and when releasing all of the linkedsmall cells in a dual connectivity environment where the macro cell andthe small cells use separated security keys according to an embodimentof the present disclosure;

FIGS. 17A and 17B are views illustrating an example of establishing adata radio bearer (DRB) path and generating and transmitting a securitykey on the control plane and user plane when adding a small cell basestation, when changing a small cell base station, and when releasing allof the linked small cells in a dual connectivity environment where themacro cell and the small cells use separated security keys according toan embodiment of the present disclosure;

FIG. 18 is a view illustrating a configuration of a user terminalaccording to an embodiment of the present disclosure; and

FIG. 19 is a view illustrating a configuration of a base stationaccording to an embodiment of the present disclosure.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components, and structures.

DETAILED DESCRIPTION

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of variousembodiments of the present disclosure as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the various embodiments describedherein can be made without departing from the scope and spirit of thepresent disclosure. In addition descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of the presentdisclosure. Accordingly, it should be apparent to those skilled in theart that the following description of various embodiments of the presentdisclosure is provided for illustration purpose only and not for thepurpose of limiting the present disclosure as defined by the appendedclaims and their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

Such denotations as “first,” “second,” “A,” “B,” “(a),” and “(b),” maybe used in describing the components of the present disclosure. Thesedenotations are provided merely to distinguish a component from another,and the essence of the components is not limited by the denotations inlight of order or sequence. When a component is described as“connected,” “coupled,” or “linked” to another component, the componentmay be directly connected or linked to the other component, but itshould also be appreciated that other components may be “connected,”“coupled,” or “linked” between the components.

Before detailing the present disclosure, examples of meanings ordenotations applicable to some terms used in this disclosure areproposed. However, it should be noted that the present disclosure is notlimited thereto.

The present disclosure targets wireless communication networks. Tasksperformed over a wireless communication network may be done while asystem (e.g., a base station) in charge of the wireless communicationnetwork controls the network and transmits data or may be done by aterminal coupled with the wireless network.

The wireless communication system includes at least one base station(BS). Each base station provides communication services within aparticular geographical area (generally referred to as a cell). A cellmay be divided into multiple areas (referred to as sectors).

A base station is an entity communicating with a terminal and may bedenoted as, e.g., a BS, a base transceiver system (BTS), a NodeB (NB),an eNodeB (eNB), or an access point (AP).

A cell should be comprehensively interpreted to denote some area coveredby a base station and collectively means a mega cell, a macro cell, asmall cell, a micro cell, a pico cell, a femto cell, or other variouscoverage areas. It should be noted that according to the context of thepresent disclosure the term “macro cell” may mean a base station of themacro cell, and the term “small cell” may mean a base station of thesmall cell.

The macro cell base station may also be referred to as a macro cell eNB,macro eNB, or MeNB.

The small cell is a cell with a smaller cell area than the macro celland may include a pico cell, a femto cell, or a micro cell. The smallcell base station may also be denoted as a small cell eNB, small eNB, orSeNB.

A user equipment is a mobile or stationary entity communicating with abase station and may be denoted as UE, mobile station (MS), mobileequipment (ME), device, wireless device, handheld device, terminal,mobile terminal (MT), user terminal (UT), or subscriber station (SS).

Downlink means communication from a base station to a terminal, anduplink means communication between a terminal to a base station. Fordownlink, a transmitter may be part of a base station, and a receivermay be part of a terminal. For uplink, a transmitter may be part of aterminal, and a receiver may be part of a base station.

FIG. 1 is a view illustrating a case in which a terminal establishesdual connectivity for a macro cell 102 and small cells that coexist in awireless communication system.

Discussion is underway for systems that offload soring mobile trafficdata by adding small cell networks to a wireless communication system(e.g., a macro cell network). As an example, the coverage of a macrocell 102 that is served by a macro cell base station 100 is denoted insolid lines, and coverages 112 and 122 of small cells that are served bysmall cell base stations 110 and 120 are circled in dotted lines asshown in FIG. 1.

The small cells 112 and 122 including at least one pico cell, femtocell, or micro cell have smaller coverage but may have multiple smallcell base stations installed therein, and thus, the small cells may playa role to offload soring mobile data. The small cells have a smallertransmission distance and good channel environment and they may thusprovide services to users at a higher data rate and may easily recyclelimited frequency resources (frequency bands) while saving powerconsumed by the terminal.

The small cells may be confronted with frequent handoff when supportingthe mobility of terminals 114 and 124 due to their smaller coverage. Tosupport such frequent handoff, the terminal needs to simultaneouslyconnect to the macro cell base station. Further, one terminal may beserved by multiple small cell base stations.

Hereinafter, dual connectivity refers to a network structure in which aterminal is served from two or more base stations connected thereto. Theterminal may be served by a base station through a control channel ordata channel and may be provided expanded services from multiple basestations without limited to those from two cells (the macro cell and thesmall cell).

FIG. 2 is a view illustrating a network control plane and user planeunder a dual connectivity situation where a macro cell and a small cellare simultaneously connected to a terminal in a wireless communicationsystem.

Referring to FIG. 2, under the dual connectivity situation where aterminal is simultaneously connected to a macro cell and a small cell,the user terminal 200 is linked to the macro cell base station (macrocell eNB (MeNB)) 202 and the small cell base station (small cell eNB(SeNB)) 204 through the connection of a network control lane or userplane. The macro cell base station 202 and the small cell base station204 may be connected through, e.g., an X2 interface 206.

Under the dual connectivity situation where the small cell is added tothe existing macro cell (e.g., the cellular network), the terminal isnot controlled by the macro cell alone. That is, under the dualconnectivity situation, the terminal may also be controlled (e.g.,resource allocation) by one or more small cell base stations.

A protocol stack structure in which the small cell base stations, aswell as the macro cell base station, also has an independent PDCP tosupport dual connectivity according to an embodiment of the presentdisclosure is described with reference to FIGS. 3 to 5.

FIG. 3 is a view illustrating an example of a protocol stack structurefor supporting dual connectivity according to an embodiment of thepresent disclosure.

The macro cell base station 300 and the small cell base station 310,respectively and independently, include packet data convergence protocol(PDCP) layers 302 and 312, radio link control (RLC) layers 304 and 314,medium access control (MAC) layers 306 and 316, and physical (PHY)layers 308 and 318.

Referring to, FIG. 3 exemplifies the structure in which the user planeof the small cell base station 310 is directly connected to a corenetwork (CN) via a separate S1 interface 320 distinguished from the userplane S1 interface 330 of the macro cell base station 300.

The small cell base station 310 is directly connected to the corenetwork (CN) via the S1 interface 320, and the small cell base station310 may transmit user plane data via a separate path, not through themacro cell base station or inter-base station connection (e.g., the X2interface).

FIG. 4 is a view illustrating another example of a protocol stackstructure for supporting dual connectivity according to an embodiment ofthe present disclosure.

Referring to FIG. 4, the macro cell base station 300 and the small cellbase station 310, respectively and independently, include PDCP layers302 and 312, RLC layers 304 and 314, MAC layers 306 and 316, and PHYlayers 308 and 318.

In particular, FIG. 4 exemplifies the structure in which the user planeof the small cell base station 310 is connected to the core network (CN)via the macro cell base station 300 through the Xn interface (e.g., theX2 interface) 420.

In such case, since the data of all the user planes is transmitted viathe macro cell base station through the inter-base station connection(X2), the transmission capacity may be limited by the latency andrestricted capacity of the backhaul. That is, the connection structureshown in FIG. 4 is a structure in which one DRB for small cells isserved through one base station (any one of the macro cell base stationor small cell base stations) without occurrence of DRB split for smallcells.

FIG. 5 is a view illustrating another example of a protocol stackstructure for supporting dual connectivity according to an embodiment ofthe present disclosure.

Referring to FIG. 5, the macro cell base station 300 and the small cellbase station 310 respectively and independently include PDCP layers 502and 302 and a PDCP layer 312, RLC layers 504 and 304 and an RLC layer314, MAC layers 506 and 306 and an MAC layer 316, and PHY layers 508 and308 and a PHY layer 318, and the user plane of the small cell basestation 310 is connected to the small cell base station via the macrocell base station from the CN.

In such case, since the data of all the user planes is transmitted viathe macro cell base station through the inter-base station connection(the Xn interface 520), the transmission capacity may be limited by thelatency and restricted capacity of the backhaul. The connectionstructure shown in FIG. 5 is a structure in which a DRB split for smallcells occurs (a split from the S1 interface 500 to the Xn interface520), and one DRB is served through multiple base stations (the macrocell base station and small cell base stations).

Now described is a scheme for generating, managing, and operating asecurity key when a macro cell and a small cell respectively includeindependent PDCP layers as described above in connection with FIGS. 3 to5.

FIG. 6 is a view illustrating an example of communication between amacro cell base station, a small cell base station, and a user terminalin a protocol layer structure where a macro cell and a small cell havean independent PDCP layer according to an embodiment of the presentdisclosure.

Data of the control plane is transferred through the macro cell basestation 300 (a link 600) in the embodiment of FIG. 6. Further, data ofthe user plane is served to the user terminal 630 through two DRBs,i.e., one through the macro cell base station 300 (particularly betweenEPS bearer 1 and the PDCP 612 of the link 602) and the other through thesmall cell base station 310 (particularly between EPS bearer 2 and thePDCP 614 of the link 604).

The user terminal 630 forms links 600 and 602 with the macro cell basestation 300 and a link 604 with the small cell base station 310 andcommunicate data via separate protocol layers (PHY, MAC, RLC, and PDCPlayers).

The PDCP layers 610, 612, and 614 independently present in the macrocell and the small cell are in charge of security. In this case,required is a design as to whether the same or separate and independentsecurity keys are used for the link 602 through the macro cell in thePDCP layer 612 of the macro cell and the link 604 through the small cellin the PDCP layer 614 of the small cell.

FIG. 7 is a view illustrating a structure of a security key used in a3GPP LTE system according to an embodiment of the present disclosure.

A lower level key may be generated from a higher level key.

The top level key K (700) used to authenticate the user terminal ispresent in the universal subscriber identity module (USIM) orauthentication center (AuC).

The lower level keys CK, IK (702) that may be generated from the higherlevel key K (700) may be known only to the UE or the home subscriberserver (HSS). That is, the lower level keys CK, IK (702) is present inthe UE or the HSS.

The MME may generate K_(ASME) (704), which is a key of the accesssecurity management entity (ASME), based on the lower level key CK, IK(702). The ASME is an entity receiving the top level key of the accessnetwork from the HSS, and for the evolved universal mobiletelecommunications system terrestrial radio access network (E-UTRAN),the MME may correspond to the ASME.

The UE or the base station may generate an integrity key K_(NASint)(708) and an encryption key K_(NASenc) (706) for the non access stratum(NAS) based on K_(ASME) (704). Further, the UE or base station maygenerate K_(eNB) 710 which is a base key for the security key of eachbase station.

The UE or the base station may generate K_(RRCint) (716), K_(RRCenc)(714), and K_(UPenc) which are security keys for the access stratum (AS)based on K_(eNB) (710).

Subsequently, generation and transfer of a security key are described,focusing on the authentication process.

UE-MME LTE mutual authentication is carried out via an evolved packetsystem authentication and key agreement (EPS AKA) procedure performedbetween the UE, the MME, and the HSS as mutual authentication betweenthe UE and the network. In the EPS AKA procedure, the HSS transmits anauthentication vector (AV) to the MME, and the MME and the UE mayperform mutual authentication using the authentication vector. As aresult of the authentication, the UE and the MME share K_(ASME) (704)and obtain, from K_(ASME) (704), K_(NASint) (708) and K_(NASenc) (706),which are security keys of the NAS, and K_(eNB) (710), which is the basekey of the base station.

It is K_(ASME) (704) that the MME receives from the HSS. Since K_(ASME)(704) cannot be transferred to the UE via the E-UTRAN, K_(ASME) (704)may be identified by KSI_(ASME) that corresponds to K_(ASME) (704) in aone-to-one manner and replaces K_(ASME) (704).

An integrity check and encryption may be performed on the NAS signalingmessage that is the control plane protocol between the UE and the MME.The integrity check is a mandatory function, and encryption is anoptional function. The base key for NAS security is K_(ASME) (704) thatis positioned in the UE and the MIME and is obtained throughauthentication between the subscriber and the network. NAS security keysare obtained from K_(ASME) (704) in the UE and the MME and come in suchtypes as integrity keys K_(NASint) (708) and encryption keys K_(NASenc)(706).

The control plane between the UE and the eNB performs an integrity check(mandatory) and encryption (optional) on RRC signaling, and the userplane performs encryption (optional) on IP packets. For access stratum(AS) security, the base key is K_(eNB) (710), and K_(eNB) (710) ispositioned in the UE and the eNB and is obtained from K_(ASME) (704).The eNB does not include K_(ASME) (704). Thus, the MME generates K_(eNB)(710) from K_(ASME) (704) and transfers to the eNB.

AS security keys are obtained from K_(eNB) (710) in the UE and the eNBand come in such types as K_(RRCint) (716), K_(RRCenc) (714), andK_(UPenc) (712). K_(RRCint) (716) and K_(RRCenc) (714), respectively,are used for integrity check and encryption on RRC signaling, andK_(UPenc) (712) is used for encryption on user plane data (IP packets).

The base station (macro cell base station or small cell base station)may receive a next hop (NH) 718 and a next hope chaining counter (NCC)720 from the MME. When the NCC 720 is transferred to the terminal, theterminal may determine the NH 718 using the NCC 720. The terminal or thebase station may generate KeNB* (722) that is a security key of the basestation using the NH 718 and may apply the generated security key as thesecurity key of the base station. KeNB* (722) may be generated based onthe NH 718 or may be generated based on a previous security key 710 ofthe base station.

FIG. 8A is a view illustrating an example of generating a security keyand transmitting related information when handover occurs between basestations in a communication system according to an embodiment of thepresent disclosure.

FIG. 8B is a flowchart illustrating an example of generating a securitykey and transmitting related information when handover occurs betweenbase stations in a communication system according to an embodiment ofthe present disclosure.

If the user terminal 800 reports a result of channel measurement(measurement report) to the serving base station 810, the serving basestation 810 or the MIME 830 determines to hand the user terminal 800over to the target base station 820 based on the channel measurementresult at operation 801.

The serving base station 810 generates a base station security key,KeNB*, to be used in the new base station (target base station) 820 atoperation 802. That is, the procedure of generating (or regenerating)the base station security key KeNB* to be used in the target basestation 820 may be initiated by the serving base station 810 or the MME830.

The serving base station 810 forwards KeNB* and the NCC to the targetbase station 820 at operation 803.

The target base station 820 informs the user terminal 800 of the NCC(804), and the user terminal 800 generates KeNB* based on the NCC atoperation 805.

The user terminal 800 and the target base station 820 perform datatransmission by applying the new base station security key KeNB* atoperation 806.

Selectively, the target base station 820 may also receive a new NH-NCCpair (i.e., {NH, NCC}) from the MIME 830 in preparation for nexthandover at operation 807.

Meanwhile, the communication network including a small cell may beinstalled so that the coverage of the small cell overlaps the coverageof the macro cell within the macro cell coverage, and the user terminalmay establish a link with each of the macro cell base station and thesmall cell base station at the same time (i.e., two or more links).Although handover occurs between equivalent base stations in the 3GPPLTE system, the small cell network according to an embodiment of thepresent disclosure overlaps the macro cell, and thus, the small celllink may be added to the link that is served only by the macro cell, thesmall cell link may be released, or the small cell link may be replacedwith a link with a new small cell base station (by handover between thesmall cells).

If the same security key as the one used in the macro cell is used forsmall cells subjected to RRC control by the macro cell, the complexityof generation of security keys may be reduced (because there is nocomputation for generating security keys for small cells), but if thesecurity keys of the small cells that are relatively security vulnerableleak, the security key of the macro cell would be highly likely to leakas well. By contrast, if the macro cell and the small cell use separateindependent security keys, they may have robust security but may causecomplicated control and overhead in generating and managing securitykeys. Receiving security keys from a higher network (e.g., the MME)every handover or when installing a few tens or a few hundreds of smallcells to increase network cell capacity would cause control overhead andlatency issues.

Accordingly, the present disclosure proposes a scheme for independentlygenerating and managing each security key or sharing and managing asingle security key according to a predetermined standard or conditionin a network where a macro cell and small cells coexist.

Further, the present disclosure proposes a scheme for generating,transmitting, and managing a security key for a transmission link basedon a security key corresponding to another transmission link and ascheme in which each base station directly generates and manages asecurity key.

In other words, there are proposed a scheme for generating and managinga security key considering layers between a macro cell and a small cellin generating and managing a security key per network layer, a schemefor generating, sharing, and using the same security key between thenetwork layers of the macro cell link and the small cell link, a schemefor generating and managing separate security keys between the networklayers of the macro cell link and the small cell link, a scheme forgenerating a security key for the small cell link based on a securitykey of the macro cell link, transmitting the security key for the smallcell link to the small cell base station, and managing the same, and ascheme for generating a security key for the small cell linkindependently from a security key of the macro cell link, transmittingthe security key to the small cell base station, and managing the same.

FIG. 9A is a view illustrating an example of generating a security keyand transmitting related information for maintaining a separate securitykey when a small cell is added according to an embodiment of the presentdisclosure.

FIG. 9B is a flowchart illustrating an example of generating a securitykey and transmitting related information for maintaining a separatesecurity key when a small cell is added according to an embodiment ofthe present disclosure.

If the user terminal 900 reports a result of channel measurement(measurement report) to the macro cell base station 910, the macro cellbase station 910 determines whether to add the small cell base station920 to the user terminal 900 based on the channel measurement result atoperation 901.

The macro cell base station 910 or the small cell base station 920 maygenerate a security key, KeNB*, of the small cell base station 920(902). That is, the procedure of generating (or regenerating) the basestation security key KeNB* to be used in the small cell base station 920may be initiated by the macro cell base station 910 or the small cellbase station 920. Selectively, the procedure of generating (orregenerating) the base station security key KeNB* to be used in thesmall cell base station 920 may also be initiated by the procedure ofvarying (or regenerating) the base station security key used in themacro cell base station 910 (the variation or regeneration of the macrocell base station security key may be initiated in the MIME or the macrocell).

The security key of the small cell base station 920 may be used by theuser terminal 900 for a new link for connection with the small cell basestation 920. Selectively, the macro cell base station 910 may inquireand obtain information to generate a base station security key to beused for the new link from the MME 930.

The macro cell base station 910 forwards the generated KeNB* and NCC tothe added small cell base station 920 at operation 903.

The macro cell base station 910 informs the user terminal 900 ofsecurity key-related information (e.g., the NCC) at operation 904, andthe user terminal 900 generates a base station security key KeNB* to beused for the link with the small cell base station based on the securitykey-related information.

Thereafter, the user terminal 900 and the added small cell base station920 may perform any DRB transmission by applying the security key KeNB*that they generated on their own or received at operation 905.

Embodiments of sharing security key information considering therelationship between base stations forming links with a user terminalare now described.

According to an embodiment of the present disclosure, the security keyof the macro cell base station may be shared by the small cell basestation.

In such case, the small cell base station may generate and use thesecurity key of the small cell base station using (based on) the shared(transferred) security key of the macro cell base station.

Further, the small cell base station may transmit the security key ofthe macro cell base station or the generated security key of the smallcell base station to another small cell base station so that the othersmall cell base station by itself may generate and use its security key.

Meanwhile, the security key of the other small cell base station mayalso be generated by the macro cell base station. That is, the securitykey of the small cell base station may be transferred (shared) to themacro cell base station, and the macro cell base station receiving thesecurity key of the small cell base station may generate the securitykey of the other small cell base station using (based on) the securitykey of the small cell base station and transfer to the other basestation so that the other small cell base station may use the same.

According to another embodiment of the present disclosure, the securitykey of the macro cell base station might not be shared by the small cellbase station.

In this case, the small cell base station receives and uses the securitykey (not the security key of the macro cell base station) of the smallcell base station that is generated and transferred by the macro cellbase station.

The security key of the small cell base station may be shared or not bythe macro cell base station.

In case the security key of the small cell base station is shared by themacro cell base station, the macro cell base station generates thesecurity key of the other small cell base station using (based on) theshared security key of the small cell base station and transmits thegenerated security key of the other small cell base station so that theother small cell may use it.

Unless the security key of the small cell base station is shared by themacro cell base station, the small cell base station generates thesecurity key of the other small cell base station and transmits thesecurity key to the other small cell base station, or the macro cellbase station generates a security key independent from the security keyof the small cell base station and transmits the independent securitykey to the other small cell base station so that the other small cellbase station may use it.

FIG. 10 is a view illustrating the process of generating andtransmitting a security key and an initial setup process of a small celllink when a macro cell determines to add a small cell on the controlplane according to an embodiment of the present disclosure.

If the user terminal 900 reports a result of channel measurement(measurement report) to the macro cell base station 910 (1002), themacro cell base station 910 determines whether to additionally link thesmall cell base station to the user terminal 900 based on the channelmeasurement result and generates a base station security key KeNB* to beused for the new link to be connected to the small cell base station 920(1004).

The macro cell base station 910 includes KeNB* and/or NCC in a smallcell add request message (SCELL ADDITION REQUEST) and forwards the sameto the small cell base station 920 (1006).

The added small cell base station 920 responds by transferring a smallcell add acknowledgement message (SCELL ADDITION ACK) to the macro cellbase station 910 (1008). Selectively, the small cell add acknowledgementmessage may contain an NCC value.

The macro cell base station 910 transfers a message for RRCreconfiguration (rrcConnectionReconfiguration) to the user terminal 900(1010). Selectively, the message for RRC reconfiguration may contain theNCC value transferred from the small cell base station 920.

Having received security key-related information (e.g., the NCC) throughthe RRC reconfiguration message 1010, the user terminal 900 may generatea base station security key KeNB* for a new link to be connected withthe small cell base station 920 (1012).

The macro cell base station 910 receives a message(rrcConnectionReconfigurationComplete) responding to the RRCreconfiguration message from the user terminal 900 (1014).

The macro cell base station 910 transmits the RRC reconfigurationmessage rather than sending out an RRC release message or RRC resetmessage to the user terminal 900 in order to prevent data transferred tothe user terminal 900 from being lost while forwarded from the macrocell base station 910 to the small cell base station 920. That is, useof the RRC release message or RRC reset message may cause the COUNTvalue forwarded to identify the user data to be initialized, thusleading to data loss. Accordingly, upon RRC configuration, an RRCreconfiguration process is performed to maintain the COUNT value. TheCOUNT value is a value recorded in the PDCP layer and is an index toidentify data transferred to the user terminal. For example, the COUNTvalue may be a value ranging from 0 to 500 and may be used forretransmission of data that may be lost upon data forwarding.

Selectively, the macro cell base station 910 may perform a process 1016of transferring the COUNT value through, e.g., a sequence number statustransfer (SN STATUS TRANSFER) message from the previous transmissionlink (i.e., the macro cell base station) to the target transmission link(i.e., the small cell base station) before forwarding the data to thesmall cell base station 920.

Subsequently, the macro cell base station 910 may forward user data tobe transferred to the user terminal 900 to the small cell base station920 (1018). Selectively, the small cell base station 920 may performmanagement to detect or prevent data loss during the process 1018 offorwarding data from the macro cell base station 910 to the small cellbase station 920 by using the COUNT value included in the sequencenumber status transfer (SN STATUS TRANSFER) message 1016.

Then, the user terminal 900 may perform transmission of a DRB with thesmall cell base station 920 based on the newly generated security keyKeNB*. Selectively, the process of transmitting the DRB may include oneor more of random access of the user terminal 900 to the small cell basestation 920 (1020), PDCP status reporting (1022), applying the newlygenerated security key KeNB* to the newly generated data radio bearer(denoted “DRB 2”) (1024), and transmitting a physical downlink sharedchannel (PDSCH) and/or physical uplink shared channel (PUSCH) for theDRB 2 (1026).

FIG. 11 is a view illustrating a process of discarding a security keyand a process in which a terminal resumes communication with a macrocell base station when the macro cell determines to release a small cellon the control plane according to an embodiment of the presentdisclosure.

If the user terminal 900 reports the channel measurement result(measurement report) to the macro cell base station (macro ENB) 910(1102), the macro cell base station 910 determines whether to releasethe link to the small cell base station 920 connected with the userterminal 900 (i.e., the small cell link) based on the channelmeasurement result and determines whether to apply the data radio bearerDRB2 used to be connected to the small cell base station 920 back toitself (i.e., the macro cell base station 910) (1104).

The macro cell base station 910 transmits a small cell release request(SCELL RELEASE REQUEST) message to the small cell base station 920(1106), and the small cell base station 920 responds by transmitting asmall cell release acknowledgment (SCELL RELEASE ACK) message to themacro cell base station 910 (1108).

Selectively, the small cell base station 920 may perform a process 1110of transferring a COUNT value to identify user data to the macro cellbase station 910 through a sequence number status transfer (SN STATUSTRANSFER) message before forwarding data to the macro cell base station910 as the small cell link is released.

Subsequently, the small cell base station 920 may forward user data tobe transferred to the user terminal 900 to the macro cell base station910 (1112). Selectively, the macro cell base station 920 may performmanagement to detect or prevent data loss during the process 1112 offorwarding data from the small cell base station 920 to the macro cellbase station 910 by using the COUNT value included in the sequencenumber status transfer (SN STATUS TRANSFER) message 1110.

The macro cell base station 910 transmits, to the user terminal 900, amessage (rrcConnectionReconfiguration) for RRC reconfiguration (1114)and receives an RRC reconfiguration complete message(rrcConnectionReconfigurationComplete) from the user terminal 900(1116). Selectively, the RRC reconfiguration message 1114 may contain anNCC value.

The macro cell base station 910 transmits the RRC reconfigurationmessage rather than an RRC release message or RRC reset message to theuser terminal 900 for RRC configuration in order to continuouslymaintain the COUNT value.

The user terminal 900 may discard the security key KeNB* generated forthe small cell link and applies the security key KeNB of the macro cellbase station to DRB 2 (1118) and may perform DRB 2 PDSCH and/or PUSCHtransmission (1120).

The macro cell base station or the small cell base station maypreviously receive multiple security key-related information items froma higher network entity (e.g., the MME) upon generating independentsecurity keys and may use the information items to generate securitykeys.

At this time, the macro cell base station may access the MME and receivemultiple security key-related information items (seed) to generate asecurity key and may transmit the security key to the small cell link.Further, a representative small cell base station may directly accessthe ME to receive multiple security key-related information items (seed)and forward them to a neighboring small cell base station, or each smallcell base station may directly access the MIME to receive securitykey-related information (e.g., seed) to generate an independent securitykey.

A base station should receive an NH from the MME in order to generate anew next hope (NH)-based security key that is not based on the securitykey KeNB of the macro cell base station. A process for the same isdescribed with reference to FIGS. 12 and 13.

FIG. 12 is a view illustrating a procedure in which a base stationobtains information for generating a security key from an MME accordingto an embodiment of the present disclosure.

Referring to FIG. 12, an NH/NCC request (NH/NCC REQUEST) message is usedas a new message to receive the NH/NCC from the MME.

If the macro cell base station 910 determines to generate a security keywhich requires a new NH-NCC pair (i.e., generating a vertical key)(1202), the macro cell base station 910 transmits an NH/NCC request(NH/NCC REQUEST) message to the MME 930 (1204). The MME 930 responds bysending out an NH/NCC response (NH/NCC RESPONSE) message to the macrocell base station 910 (1206).

Here, the NH/NCC response message 1206 may contain the NH-NCC pair.Selectively, the NH/NCC response message 1206 may include a plurality ofNH-NCC pairs {NH, NCC}. Since the small cell base station has small cellcoverage, handover or adding, release, or change of small cells may befrequent, and thus, more NH-NCC pairs {NH, NCC} are required forgenerating security keys for base stations.

Although the macro cell base station is an entity to send out the NH/NCCrequest message 1204 to request {NH, NCC}, for example, the small cellbase station may, in some cases, send out the message to obtain {NH,NCC} from the MME.

FIG. 13 is a view illustrating another procedure in which a base stationobtains information for generating a security key from an MME accordingto an embodiment of the present disclosure.

A path switch request (PATH SWITCH REQUEST) message is a messagetransmitted from a base station to the MME to change or switch datatransmission paths (PATH) when handover occurs between base stations. Inthe embodiment shown in FIG. 13, the path switch request (PATH SWITCHREQUEST) message is used to receive the NH/NCC from the MME.

If the macro cell base station 910 determines to generate a security keywhich requires a new NH-NCC pair (i.e., generating a vertical key)(1302), the macro cell base station 910 transmits the path switchrequest (PATH SWITCH REQUEST) message to the MME 930 (1304).

In case a small cell base station is added, released, or changed, thedata transmission path to the small cell base station may be varied ornot depending on the structure of the user plane.

As an example, since the data transmission path interface 320 of thesmall cell becomes different from the data transmission path interface330 of the macro cell in the case shown in FIG. 3, a path switch occurs.

As another example, in the case shown in FIGS. 4 and 5, although a smallcell is added, the data transmission path is not changed (the same pathpassing through the macro cell as the one before the small cell isadded). In such case, the path switch request message may contain, e.g.,the same transmission layer address as the present one and E-radioaccess bearer (E-RAB) information.

Unless the path switch request (PATH SWITCH REQUEST) message receivedfrom the macro cell base station 910 indicates a path switch or change(e.g., the case shown in FIGS. 4 and 5), the MME 930 performs onlyoperations regarding the NH and NCC without a path switch, while in casethe path switch request (PATH SWITCH REQUEST) message indicates a pathswitch, the MME 930 performs operations regarding the NH and NCC and adata path switching operation (1306). That is, the MME 930 transmits apath switch request acknowledgment (PATH SWITCH REQUEST ACK) messagecontaining the NH-NCC pair to the macro cell base station 910 inresponse to the path switch request message (1308).

Selectively, the path switch request acknowledgment (PATH SWITCH REQUESTACK) message 1308 may include a plurality of NH-NCC pairs {NH, NCC}.Since the small cell base station has small cell coverage, handover oradding, release, or change of small cells may be frequent, and thus,more NH-NCC pairs {NH, NCC} are required for generating security keysfor base stations.

Although the macro cell base station is an entity to send out the pathswitch request (PATH SWITCH REQUEST) message 1304 to request {NH, NCC},for example, the small cell base station may, in some cases, send outthe message to obtain {NH, NCC} from the MME.

Described is an embodiment of independently generating and managing asecurity key when a user terminal adds a link with a small cell basestation.

According to an embodiment of the present disclosure, there is proposeda scheme for selectively maintaining the independence of security keysbetween small cell base stations while managing a security key of amacro cell and security keys of small cells to be independentlygenerated. That is, the security keys between the small cells may bedetermined to be generated independently from each other in some casesor to be dependent, the same or similar to each other in other cases.

Specifically, a small cell first added to the macro cell coverage may berendered to generate a security key independent from the security key ofthe macro cell base station. A small cell added next to the first smallcell may generate a dependent security key based on the security key ofthe first small cell.

Further, under a certain condition, it may be determined to generate asecurity key independent from a small cell base station added. Forexample, the condition to generate a security key independent from asmall cell base station added may include when a cluster of small cellspositioned within a predetermined physical distance is varied or when atimer to generate an independent security key expires (i.e., apredetermined time or more elapses after a previous independent securitykey is generated).

FIG. 14 is a view illustrating a process of generating security keys fora macro cell base station, a small cell base station, and a userterminal and applying the generated security keys when the macro cellbase station determines to add a small cell according to an embodimentof the present disclosure.

Selectively, the small cell base station may be notified that small cellServCell_s is added by receiving information such as SCellToAddRemoveinformation element (IE) from the macro cell base station (1400). Here,ServCell_s means a serving cell controlled by the small cell basestation. For example, the SCellToAddRemove IE may be contained in asmall cell add request (SCELL ADD REQUEST) message.

The macro cell base station (or small cell base station or userterminal) determines whether the small cell to be added is a small cellfirst added (1402). Further, the user terminal may generate a securitykey for the small cell base station (a security key for a DRB to beadded) through operations 1410 and 1412 for determining an NCC valuetransmitted from the macro cell base station (or small cell basestation). That is, different security key generating methods may applydepending on the NCC value transmitted from the base station.

A process of generating and applying a security key is described indetail.

In case as a result of the determination 1402 the small cell is firstadded, the user terminal sets the sum of the received NCC_s value andone, i.e., NCC_s+1, to a local NCC_s value (1404). Here, NCC_s is an NCCmaintained for the security key KeNB_s of the small cell base station.The next hop chaining counter (NCC) may be represented in three bits,and at this time, may be used to distinguish the security keys for eightbase stations for one K_(ASME).

The user terminal selects the ID of the macro cell base station,physical cell ID (PCI), and operation frequency, downlink-EUTRANabsolute radio frequency channel number (DL-EARFCN), according to apredetermined rule (1406).

Subsequently, the user terminal may generate a security key KeNB_s* forthe small cell base station as in Equation 1 by applying the receivedNCC_s, the PCI, and the DL_EARFCN to a key derivation function (KDF)(1408).KeNB_s*=KDF[NH(NCC_s),PCI,DL-EARFCN]  Equation 1

Here, NH(NCC_s) is a function to calculate next hop (NH) using NCC_s.

As such, the method of generating a new security key using the NH valueto independently maintain the security keys of the macro cell basestation and the small cell base station is called vertical security keyderivation. That is, when the received NCC value differs from theprevious NCC (local NCC), it may be determined to generate a newsecurity key according to the vertical security key derivation method.

If it is determined in 1402 that the small cell is not first added(i.e., when there are one or more small cells already added), the userterminal determines whether the NCC_s has been signaled from the macrocell base station (1410).

In case it is determined in 1410 that the NCC_s has not been signaled,the user terminal may determine to use (recycle) the existing securitykey of the small cell base station without generating a security key(1424).

Thus, the user terminal applies the security key KeNB_s for a logicalchannel LCH_s served by the small cell and applies the security keyKeNB_m for a logical channel LCH_m served by the macro cell (1422).

Meanwhile, in case it is determined in 1410 that the NCC_s has beensignaled, the user terminal determines whether the signaled (received)NCC_S is the same as the local NCC_S (1412).

In case a result of the 1412 determination indicates “the same,” theuser terminal may select a PCI and DL-EARFCN according to apredetermined rule (1416) and may apply the present security key KeNB_sof the small cell base station, the PCI, and the DL-EARFCN to the keyderivation function to generate a security key KeNB_s* for the smallcell base station as in Equation 2 (1418).KeNB_s*=KDF[KeNB_s,PCI,DL-EARFCN]  Equation 2

The method of generating a new security key based on an existing basestation security key as shown in Equation 2 is called a horizontalsecurity key derivation method. That is, when the received NCC value isthe same as the previous NCC (local NCC), it may be determined togenerate a new security key according to the horizontal security keyderivation method.

In other words, even though the small cell base station (or the macrocell base station) does not receive control information (e.g., NH orNCC) to generate a new security key from the MME, the user terminalgenerates a new security key based on the existing small cell basestation security key. This way may reduce overhead that may be caused bycontrol signaling in the MME due to frequent security key generationtasks.

Unless the result of the 1412 determination indicates “the same,” theuser terminal sets the received NCC_s value to the local NCC_s value(1414) and performs the selection of PCI and DL-EARFCN (1406) andgeneration of a key using the NCC_s (1408).

If the security key of the small cell base station is generated throughthe operation 1418 or 1408, the user terminal sets the generated basestation security key KeNB_s* to the new base station security key KeNB_s(1420) and may apply the security key to the logical channel for datatransmission (1422).

Selectively, independent security keys respectively for the macro cellbase station and the small cell base station may be generated accordingto a determination by the macro cell or a rule under a particularcondition or need. For example, the macro cell base station or the smallcell base station (or the user terminal) may operate a security keygeneration timer to perform management so that if a predetermined timeelapses, security keys may be independently and periodically generatedor may perform management so that independent security keys may begenerated when the number of small cell base stations recycling securitykeys exceeds a predetermined number.

FIG. 15 is a view illustrating a process of generating security keys fora macro cell base station, a small cell base station, and a userterminal and applying the generated security keys when the macro cellbase station determines to release or replace a small cell according toan embodiment of the present disclosure.

Selectively, the small cell base station may be notified that small cellServCell_s is released by receiving information such as SCellToAddRemoveIE from the macro cell base station (1500). For example, theSCellToAddRemove IE may be contained in a small cell release request(SCELL RELEASE REQUEST) message.

The macro cell base station (or small cell base station or userterminal) determines whether the small cell to be released is a smallcell released last (1502). Further, the user terminal may determine abase station security key to be used through the operation ofdetermining an NCC value transmitted from the macro cell base station(or the small cell base station). That is, different base stationsecurity key applying methods may apply depending on the NCC valuetransmitted from the base station.

A process of generating and/or applying a security key is described indetail.

In case it is determined in 1502 that the small cell is released last,the user terminal releases the existing security key KeNB_s of the smallcell base station and resets (i.e., initializes) the local NCC_s value(1504). Accordingly, the security key of the macro cell base station,KeNB_m, may apply to both the logical channel LCH_m served by the macrocell and the logical channel LCH_s served by the small cell (i.e., thelogical channel to replace the released small cell) (1506). Here,although the logical channel when the small cell is released is denotedas LCH_s for convenience, the logical channel LCH_s with the small cellbase station released (accordingly not present any longer) should beinterpreted to mean a logical channel with the macro cell base station(however it is denoted).

If it is determined in 1502 that the small cell is not released last(i.e., when there are one or more small cells left after the release),the user terminal determines whether the NCC_s has been signaled fromthe macro cell base station (1508).

In case it is determined in 1508 that the NCC_s has not been signaled,the user terminal does not generate a security key. The user terminaldetermines to use (i.e., recycle) the existing security key of the smallcell base station as a base station security key to replace the releasedsmall cell (1514).

In case it is determined in 1508 that the NCC_s has been signaled, theuser terminal generates a security key of the small cell base station toreplace the released small cell using the vertical security keyderivation method or horizontal security key derivation method accordingto the signaled (or received) NCC_s value similar to when a small cellis added (1510).

If the security key of the small cell base station is determined(generated) according to operation 1514 or 1510, the security key KeNB_mof the macro cell base station applies to the logical channel LCH_mserved by the macro cell, and the determined (generated) security keyKeNB_s of the small cell base station applies to the logical channelLCH_s served by the small cell (i.e., the logical channel with the basestation to replace the released small cell) (1512).

FIGS. 16A and 16B are views illustrating an example of generating andtransferring a security key when adding a small cell base station, whenchanging a small cell base station, and when releasing all of the linkedsmall cells in a dual connectivity environment where the macro cell andthe small cells use separated security keys according to an embodimentof the present disclosure.

The embodiment shown in FIGS. 16A and 16B is regarding a scenario inwhich the user terminal initially receiving two DRBs through the macrocell base station using the security key KeNB of the macro cell basestation receives one of the DRBs (referred to as DRB 2 and indicatingthe LCH_s as shown in FIGS. 16A and 16B) through a small cell basestation 1 added and then hands over to a small cell base station 2 forthe DRB 2 and finally releases the link with the small cell base station2 to receive both the DRBs from the macro cell base station.

The security key KeNB_m of the macro cell base station may be obtained,as in Equation 3, using a key derivation function whose input valuesinclude the macro cell base station ID, which is the physical cell ID(PCI), the operation frequency, which is the downlink-EUTRAN absoluteradio frequency channel number (DL_EARFCN), and the next hop (NH)(1610).KeNB_m=KDF[NH(n),PCI,DL-EARFCN]=K1   Equation 3

Initially, the user terminal 1600 is served two DRBs (DRB 1 indicatingLCHC_m and DRB 2 indicating LCH_s) by the macro cell base station 1602,and the security key of the macro cell base station is commonly used forthe two DRBs (1612 and 1614).

In case a variation in channel status is detected based on a channelmeasurement report of the user terminal, the macro cell base station1602 adds a new small cell and determines to connect one (here, DRB 2)of the user DRBs to the small cell base station 1604 (1616). The macrocell base station 1602 generates an NH value for use in verticalsecurity key generation (1618).

At this time, the security key for the new small cell base station 1604is obtained as in Equation 4 using the KDF whose input values includethe PCI, which is the ID of the small cell base station, the DL-EARFCN,which is the operation frequency, and the new NH value (1620). That is,the security key for the new small cell base station 1604 may begenerated by the vertical security key derivation method.KeNB_s*=KDF[NH(n+1),PCI,DL-EARFCN]=K2   Equation 4

Here, n is the value of NCC_m which is an NCC maintained by the securitykey of the macro cell base station.

If the macro cell base station 1602 transmits the security key KeNB_s*generated for the small cell base station to the small cell base station(1656), the small cell base station applies the received KeNB_s* as asecurity key of the new base station.

The macro cell base station transmits the NCC_s information to the userterminal 1600 as well (1658). The user terminal 1600 may restore(generate) and use the security key KeNB_s* for the DRB 2 connected tothe small cell base station 1 1604 based on the received NCC_s.

Specifically, the user terminal 1600 makes comparison as to the receivedNCC_s (1622). The NCC_s is initialized to the local NCC when handoveroccurs (or the initial small cell is added), and in case the new NCC_svalue is signaled, it is updated with the received NCC_s value. When thereceived NCC_s value differs from the local NCC_s value, the userterminal 1600 may generate an NH with the received NCC_s value (1624)and may generate a value K2 of the security key KeNB_s of the small cellbase station using the PCI and the DL-EARFCN values (1626). Then, thesecurity key KeNB_m may apply to DRB 1 (1628), and the security keyKeNB_s may apply to DRB 2 (1630).

Next, the channel condition changes, and the macro cell base station1602 determines to switch (i.e. handover) the DRB 2 connection of theuser terminal 1600 from the small cell base station 1 1604 to the newsmall cell base station 2 1606 (1632). At this time, the security keyfor the small cell base station 2 1606 is obtained as in Equation 5using the key derivation frequency (KDF) based on the PCI, which is theID of the small cell base station 2 1606, the operation frequencyDL-EARFCN, and the security key KeNB_s of the previous small cell basestation. That is, the security key for the replacing small cell basestation 1606 may be generated by the horizontal security key derivationmethod (1634).KeNB_s*=KDF[KeNB_s,PCI,DL-EARFCN]=K3   Equation 5

The macro cell base station 1602 may transmit the security key KeNB_s*(K3) generated for the small cell base station 2 1606 to the small cellbase station 2 1606 (1660). Further, the macro cell base station 1602transmits the NCC_s information to the user terminal 1600 as well(1662). The user terminal 1600 may make comparison as to the receivedNCC_s (, SeNB_NCC), and restore (generate) and use the security keyKeNB_s* for the DRB 2 connected to the small cell base station 2 1606based on the NCC_s (1636 and 1638). As a result, the user terminal mayapply the base station security keys for DRB 1 and DRB 2 (1640 and1642).

Next, the channel condition changes, and if the macro cell base station1602 determines to release the DRB 2 which is the connection of the userterminal 1600 to the small cell base station 2 1606 and to connect thesame to the macro cell base station 1602 (hand over) (1644), the macrocell base station 1602 transmits a small cell release (SCELL RELEASE)message to the user terminal 1600 (1646).

When releasing the last small cell base station (1648) (i.e., when thereis no more serving cell to be linked to the LCH_s), the macro cell basestation 1602 discards the security key KeNB _s of the small cell basestation 2 that has been used and resets the NCC_s (1650). It performstransmission with the security key KeNB_m (k1) of the macro cell basestation 1602 applied to DRB 2 as well as DRB 1 (1652 and 1654).

FIGS. 17A and 17B are views illustrating an example of establishing aDRB path and generating and transmitting a security key on the controlplane and user plane when adding a small cell base station, whenchanging a small cell base station, and when releasing all of the linkedsmall cells in a dual connectivity environment where the macro cell andthe small cells use separated security keys according to an embodimentof the present disclosure.

The embodiment shown in FIGS. 17A and 17B is also regarding a scenarioin which the user terminal initially receiving two DRBs through themacro cell base station using the security key KeNB of the macro cellbase station receives one of the DRBs (referred to as DRB 2 andindicating the LCH_s as shown in FIGS. 17A and 17B) through a small cellbase station 1 added and then hands over to a small cell base station 2for the DRB 2.

In case the user terminal 1700 which has initially been connected withthe macro cell base station 1702 alone determines to add a small cellbase station to receive a service (1710), the macro cell base station1702 transmits a small cell add request to the small cell base station1704 while generating and forwarding a security key KeNB_s* (1712). Thesmall cell base station 1 1704 responds to the request 1712 (1714). Themacro cell base station 1702 transmits a sequence number status transfer(SN STATUS TRANSFER) message including a COUNT value to the small cellbase station 1704 (1720), and forwards user data to be transmitted tothe user terminal to the small cell base station 1704 (1722).

The macro cell base station 1702 stops the logical channel LCH_s throughwhich the user terminal 1700 connects to the macro cell base station1702 (1716) and transmits an RRC reconfiguration message for connectionwith the small cell base station 1 1704 and the user terminal 1700 tothe user terminal 1700 while informing the user terminal 1700 of theNCC_s information (1718).

The user terminal 1700 stops the logical channel LCH_s connecting withthe macro cell base station (1724) and generates the KeNB_s based on theNCC_s information and applies the same to the DRB 2 (1726). Here,although the logical channel through which the user terminalcommunicates with the macro cell base station is denoted LCH_s forconvenience, the LCH_s when the communication with the macro cell basestation is stopped for communicating with a new small cell base stationshould be interpreted to mean a logical channel with the macro cell basestation (regardless of however it is denoted).

Subsequently, the user terminal 1700 sends an RRC reconfigurationcomplete message to the macro cell base station 1702 (1728). The userterminal 1700 starts to transmit DRB 2 with the small cell base station1704 (1732). In this case, the security key KeNB_s applies for a logicalchannel LCH_s served by the small cell, and the security key KeNB_mapplies for a logical channel LCH_m served by the macro cell (1734 and1736).

Selectively, PDCP status reporting 1738 may occur between the userterminal 1700 and the small cell base station 1704, and radioenvironment measurement reporting 1740 may occur from the user terminal1700 to the macro cell base station 1702.

Next, in case the macro cell base station 1702 determines to change thesmall cell base station to be used for DRB 2 from the small cell basestation 1 1704 to the small cell base station 2 1706 (1742), the macrocell base station 1702 sends a request message to release the small cellbase station 1 1704 to the small cell base station 1 1704 (1744). Atthis time, the small cell base station 1 1704 stops transmitting the DRB2 (1746) and sends a response to the request message 1744 to the macrocell base station 1702 (1748). At this time, the macro cell base station1702 may generate and transfer the security key KeNB_s* while sending arequest to add a small cell to the small cell base station 2 1706(1750). The small cell base station 2 1706 responds to the add request1750 (1752).

The macro cell base station 1702 performs RRC reconfiguration forconnection with the new small cell base station 2 1706 and the userterminal 1700 (1754 and 1760). The macro cell base station 1702 may passthe NCC_s over while sending the RRC connection reconfiguration message1754 to the user terminal 1700. The user terminal 1700 stops the logicalchannel LCH_s for DRB 2 (1756), generates a new small cell base stationsecurity key KeNB_s* (i.e., a horizontal base station security key)based on the received NCC_s, and applies the same to the DRB 2 (1758).

FIG. 18 is a view illustrating a configuration of a user terminalaccording to an embodiment of the present disclosure.

The UE 1800 includes a transceiver 1810 to communicate signals and acontroller 1805 to control the overall operation of the UE 1800. Thetransceiver 1810 may play a role to communicate various control signalsand data with a macro cell base station or a small cell base station.The operations of the user terminal as exemplified herein may beappreciated as performed under the control the controller 1805.

Although the transceiver 1810 and the controller 1805 are shown as ifthey are separate components, they may also be implemented in a singlecomponent.

FIG. 19 is a view illustrating a configuration of a base stationaccording to an embodiment of the present disclosure.

The base station 1900 is an example of a macro cell base station orsmall cell base station as described herein.

The base station 1900 includes a transceiver 1910 to communicate signalsand a controller 1905 to control the overall operation of the basestation 1900. The transceiver 1910 may play a role to communicatevarious control signals and data with a user terminal or a small cellbase station. The operations of the macro cell base station or smallcell base station as exemplified herein may be appreciated as performedunder the control the controller 1905.

Although the transceiver 1910 and the controller 1905 are shown as ifthey are separate components, they may also be implemented in a singlecomponent.

It should be noted that examples of inter-system signal transfer,generation of security keys, and configuration of the apparatus asillustrated in FIGS. 3 to 19 are not intended to limit the scope of thepresent disclosure. In other words, all the entities, operations orcomponents illustrated in FIGS. 3 to 19 should not be construed asessential components to practice the present disclosure, and the presentdisclosure may be rather implemented with only some of the componentswithout departing from the gist of the present disclosure.

The above-described operations may be realized by equipping a memorydevice retaining their corresponding codes in the entity, base station,or user terminal of the communication system. That is, the controller inthe entity, the base station, or user terminal may execute theabove-described operations by reading and executing the program codesstored in the memory device by a processor or central processing unit(CPU).

As described herein, various components or modules in the entity, thebase station, or the user terminal may be operated using a hardwarecircuit, e.g., a complementary metal oxide semiconductor-based logiccircuit, firmware, software, and/or using a hardware circuit such as acombination of hardware, firmware, and/or software embedded in amachine-readable medium. As an example, various electric structures andmethods may be executed using electric circuits such as transistors,logic gates, or ASICs.

While the present disclosure has been shown and described with referenceto various embodiments thereof, it will be understood by those skilledin the art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the present disclosure asdefined by the appended claims and their equivalents.

What is claimed is:
 1. A method for communicating by a user equipment(UE) in a communication system, the method comprising: receiving, from afirst base station, a counter for a second base station; generating asecond security key for a second communication link with the second basestation based on a first security key and the counter; and applying thesecond security key to the second communication link with the secondbase station.
 2. The method of claim 1, wherein the first security keyis applied to a first communication link with the first base station,and wherein the second security key is different from the first securitykey.
 3. The method of claim 1, wherein the counter is received through aradio resource control (RRC) connection reconfiguration message.
 4. Themethod of claim 1, further comprising: receiving, from the first basestation, a new counter in a case that the second base station is newlyadded; and generating a new second security key for the newly addedsecond base station based on the new counter.
 5. A user equipment (UE)communicating in a communication system, the UE comprising: atransceiver; and a controller configured to: receive, from a first basestation, a counter for a second base station; generate a second securitykey for a second communication link with the second base station basedon a first security key and the counter; and apply the second securitykey to the second communication link with the second base station. 6.The UE of claim 5, wherein the first security key is applied to a firstcommunication link with the first base station, and wherein the secondsecurity key is different from the first security key.
 7. The UE ofclaim 5, wherein the counter is received through a radio resourcecontrol (RRC) connection reconfiguration message.
 8. The UE of claim 5,wherein the controller further configured to: receive, from the firstbase station, a new counter in a case that the second base station isnewly added; and generate a new second security key for the newly addedsecond base station based on the new counter.